1. Field of the Invention
This invention relates to an ignition system for the internal combustion engine or more in particular to an ignition system having a retard circuit.
2. Description of the Prior Art
The conventional ignition systems are such that the ignition is performed by turning on and off a power transistor. With the increase in the retard angle, a first-stage transistor may sometimes cause the power transistor to turn on immediately following the turning off thereof, in which case the primary current fails to be cut off, thereby leading to the troubles of a firing failure or an interrupted firing.
For overcoming such problems, there has been provided a conventional ignition system having a retard circuit such as disclosed by U.S. Pat. No. 4,106,447, in which pulses are generated in response to an output signal (coincident with the ignition timing) of a retard circuit for a predetermined period of time during which the ignition coil is forcibly turned off.
This type of conventional ignition system requires an electromagnetic pick-up coil voltage waveform processing circuit for both the ignition system and the retard circuit, resulting in a bulky total system on the one hand and complicating the control circuit for the retard circuit on the other hand.
The U.S. patent application Ser. No. 44,998 dated Jan. 4, 1979, now U.S. Pat. No. 4,248,200, issued Feb. 3, 1981, discloses a circuit for increasing the duty factor (ignition coil conduction time/period).times.100%) depending on the r.p.m. of the engine, a circuit for limiting the current of a power transistor, a circuit for turning on the power transistor under non-saturated conditions and a circuit for reducing the duty factor depending on the non-saturation time. This particular circuit, however, does not disclose any means for preventing an ignition failure by limiting the increase of the duty factor.